Cellulose acylate films have high transparency and mechanical properties, and also have dimensional stability that is little dependent on humidity and temperature. Therefore, they are widely used as supports of optical materials requiring those features. In general, the cellulose acylate films are manufactured by casting a dope made by melting a solvent with polymer onto a continuous support.
The casting method can be largely divided into belt casting and drum casting in accordance with the types of continuous supports. The belt casting is a method that casts a dope on a belt, dries and separates a solvent, and conveys a film to the next process, while the drum casting is a method that casts a dope on a drum, separates it without drying, and conveys a film to the next process.
In general, the belt casting can manufacture a variety of films because it can control dry conditions of films, while the drum casting can be used for mass production, because it can perform high-speed casting. Both of the methods should accurately machine the surfaces of the drum and the belts, which are continuous supports, to implement a clean surface required for liquid crystal displays.
It is required to accurately polish the surfaces of both the belt and drum in order to accurately machine the surface of the continuous supports. The belt is manufactured by grinding and polishing a belt made of stainless steel while the drum is manufactured by plating nickel and hard chrome on a drum made of carbon steel and polishing it.
Although it is not a problem in the drum, referring to FIG. 1, it is required for the belt to forming a joint 11 connecting both ends of the belt to achieve a continuous support. Welding is widely used to form the joint, and TIG welding and laser welding are commonly used. The welding may be performed before polishing, and may be performed after polishing. The welded portion should be polished to prevent a problem in manufacturing a film, because the welded portion may remain on the belt after the welding. However, it is impossible to completely remove the welded portion and it is possible to see the welded portion with naked eyes. Therefore, some products are used with the welded portion cut off, when a polarizer or an optical compensation film is manufactured by using produced cellulose acylate.
Recently, the size of liquid crystal displays gradually increases, and accordingly, the width of polarizers and optical compensation films increases. Therefore, it needs to increase the width of cellulose acylate to come up with the increase in width and to increase yield in manufacturing the polarizers and optical compensation films.
Since a drum having a width of 2030 mm or more can be manufactured by technologies that have been developed at the present time, a wide film can be manufactured, whereas it is known that a belt having a width of maximum 2030 mm can be manufactured. Therefore, two belts can be longitudinally welded to achieve a wide belt having a width over 2030 mm; however, a joint is formed at the center longitudinally between two belts by welding, such that the central joint is imprinted on a manufactured film and the film cannot be used itself.
The dope casted on the belt is separated and manufactured in a film by a tender and a drier. The film contacts in the width direction from casting to separating, such that necessary extension is performed by the tender. The dimensions of the film are not largely changed in the drying process using the drier. In general, a trimming process that cuts off both ends of the film is performed to smoothly convey the film and keep the properties of the entire film, and the trimming process is performed at one to two times after the casting process, the tender process, and the drying process.
Therefore, when a belt having a width of 2030 mm is used, the width of an available film is 2030 mm or less. Considering stability, extension in the tender, and cutting-off of both ends of the film in the substantial casting, the available maximum width is 1800 mm, such that it is difficult to manufacture a film according to the increase in width.